Methods and apparatus for increased dynamic range of an image sensor

ABSTRACT

Various embodiments of the present technology may comprise methods and apparatus for increasing dynamic range of an image sensor. According to an exemplary embodiment, the image sensor comprises a backside-illuminated hybrid bonded stacked chip image senor having a pixel circuit array. A capacitor is formed on each pixel circuit along two adjacent sidewalls of an epitaxial substrate layer facing a deep trench isolation region. The capacitor may also extend along an upper surface of the epitaxial substrate layer.

BACKGROUND OF THE TECHNOLOGY

Electronic devices, such as cellular telephones, cameras, and computers,commonly use image sensors to capture images by sensing light. A typicalimager sensor includes a focal plane array of pixels, and each pixelincludes a photosensor, such as a photogate, photoconductor, orphotodiode, for accumulating photo-generated charge in a portion of thesubstrate. When photons impinge on the photosensor, electron-hole pairsare generated. Conventional image sensors convert the electrons that areintegrated (collected) in the pixels into a voltage, and the holes aregenerally discarded into the substrate.

Typical CMOS image sensors have scene capture ranges within a range of40-60 dB. This range is less than a human eye intra-scene capture range.A solution to this problem is high dynamic range (HDR) scene luminancethat has a capture range of about 60-120 dB.

Current efforts to provide HDR scene luminance include intra-framemulti-exposure and interlaced exposures and have met with varyingdegrees of success. For example, several methods being used to provideHDR suffer from high memory requirements resulting from the need tostore additional exposures, high power requirements, loss in resolution,and motion artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present technology may be derivedby referring to the detailed description when considered in connectionwith the following illustrative figures. In the following figures, samereference numbers refer to similar elements and steps throughout thefigures.

FIG. 1 representatively illustrates a cross-sectional view of aphotodiode having a capacitor extending over two sidewalls of thephotodiode in accordance with the present technology;

FIG. 2 representatively illustrates a cross-sectional view of a pair ofadjacent photodiodes each having a capacitor extending over twosidewalls of the photodiode in accordance with the present technology;

FIG. 3 representatively illustrates a simplified perspective view of anarray of photodiodes having a two sided capacitor in accordance with thepresent technology;

FIG. 4 representatively illustrates a cross-sectional view of a pair ofphotodiodes each having a capacitor extending over two sidewalls and anupper surface of the photodiode in accordance the present technology;

FIG. 5 representatively illustrates a simplified perspective view of anarray of photodiodes having a capacitor extending over two sidewalls andan upper surface of the photodiode in accordance with the presenttechnology;

FIG. 6 representatively illustrates a cross-sectional view of a pair ofadjacent photodiodes each having a capacitor extending over one wall andthe upper surface of the photodiode in accordance the presenttechnology;

FIG. 7 representatively illustrates a cross-sectional view of analternative pair of photodiodes each having a capacitor extending overtwo sidewalls and an upper surface of the photodiode in accordance thepresent technology;

FIG. 8 representatively illustrates a cross-sectional view of analternative pair of photodiodes each having a capacitor extending overtwo sidewalls and an upper surface of the photodiode in accordance thepresent technology; and

FIGS. 9A-9G illustrate a simplified formation process for a pixelcircuit of an image sensor in accordance with the present technology.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present technology may be described in terms of functional blockcomponents and various signal processing steps. Such functional blocksmay be realized by any number of components configured to perform thespecified functions and achieve the various results. For example, thepresent technology may employ various semiconductor devices,transistors, capacitors, and the like, which may carry out a variety offunctions. In addition, the present technology may be practiced inconjunction with any number of systems, such as portable devices,consumer electronics, automotive systems, surveillance systems, and thelike, and the system described is merely one exemplary application forthe technology. Further, the present technology may employ any suitablepixel architecture, readout scheme, and/or device layout.

Methods and apparatus for increasing dynamic range of an image sensoraccording to various aspects of the present technology may provideimproved pixel capacitance to allow for greater luminance range sensing.The methods and apparatus for increasing high dynamic range for an imagesensor according to various aspects of the present technology mayoperate in conjunction with any suitable image capture system, such as adigital camera, a cellular telephone, a tablet computer, a web camera, avideo camera, a video surveillance system, an automotive imaging system,a video gaming system with imaging capabilities, or any other desiredimaging system. Further, the disclosed technology for the image sensormay be utilized with any suitable imaging system, such as a camerasystem, video system, machine vision, vehicle navigation, surveillancesystem, motion detection system, and the like.

An exemplary imaging system may comprise an electronic device, such as adigital camera or digital video camera, configured to capture imagedata. For example, the imaging system may comprise a central processingunit (CPU) that communicates with various devices over a bus. Some ofthe devices connected to the bus may provide communication into and outof the system, for example an input/output (I/O) device.

The imaging system may be configured to construct a digital high dynamicrange (HDR) image using both low light level signals and high lightlevel signals from a single exposure. The imaging system may furthercomprise or be coupled to a display screen for viewing the HDR image.The display screen may be coupled to and configured to send and/orreceive information, such as image data, from the bus.

The imaging system comprises an image sensor for capturing and conveyingimage data. For example, the image sensor may comprise an array of pixelcircuits to detect the light and convey information that constitutes animage by converting the variable attenuation of photon flow (as theypass through or reflect off object) into electrical signals. The imagesensor may be implemented in conjunction with any appropriatetechnology, such as active pixel sensors in CMOS technology.

In various embodiments, the imaging system may comprise a primary lensto focus an image or scene onto the image sensor. For example, light mayenter the imaging system through the primary lens and strike the imagesensor. The image sensor may capture and generate image datacorresponding to one or more rows in the array of pixel circuits. Theimage data may comprise image pixel data corresponding to one or morepixel circuits in the array of pixel circuits. For example, image datamay comprise information representing a measured voltage, current, orother quantity as a result of light absorbed by each pixel circuit, suchthat each image pixel comprises an individual pixel intensity value.

The image sensor processes received image data according to any suitablefunctions such as demosaicing, white balancing, noise reduction, colorcorrection, and the like. The image sensor may further comprise varioussignal processing circuits and/or systems, such as sample-and-holdcircuitry, an analog-to-digital converter, an amplifier, and the like,used to convert the pixel charge to a digital image.

Referring now to FIG. 1, each pixel circuit 100 within an array of pixelcircuits is configured to capture a portion of an image or scene. Thepixel circuit 100 may be formed of a first chip and a second chipstacked vertically. The pixel circuit 100 comprises a photodetector anda capacitor configured to convert light (photons) into an electriccharge consisting of electrons and holes (electron-hole pairs), whichmay be referred to individually as electron charge and hole charge. Thephotodetector may comprise any appropriate device responsive to lightsuch as: a photodiode; a photogate; or the like.

A solid-state pixel structure corresponding to a portion of the pixelcircuit 100 comprises an epitaxial substrate layer 116 having an uppersurface covered with a silicon layer 112 (e.g. a passivation layer)formed of any suitable material such as silicon dioxide (SiO₂) orsilicon nitride (Si₃N₄), to isolate and/or protect the epitaxialsubstrate layer 116. The pixel circuit 100 may further comprise a colorfilter 108 and a microlens 110. In an exemplary embodiment, a first sideof the color filter 108 is disposed on an opposite of the silicon layer112 as the epitaxial substrate layer 116. The microlens 110 is disposedadjacent to a second side of the color filter 108 that opposes thesilicon layer 112. This arrangement is commonly referred to as abackside illuminated image sensor pixel (BSI). The microlens 110concentrates impinging photons to the photodetector and generates theelectrons and holes, which are simultaneously generated and integrated(collected) in doped regions of the epitaxial substrate layer 116.

Additional surfaces of the epitaxial substrate layer 116 may be furtherisolated by a liner comprising a first oxide layer 106 disposed along atleast one sidewall and a second oxide layer 118 disposed along the uppersurface between the epitaxial substrate layer 116 and the silicon layer112. More specifically, and referring now to FIGS. 2 and 3, within anarray of pixel circuits 300, a first epitaxial substrate layer 116 a ofa first pixel circuit 100 a may be isolated from a second epitaxialsubstrate layer 116 b of a second pixel circuit 100 b by a deep trenchisolation region 102 that is filled with a conductive material 104 suchas: a doped polysilicon; copper; Tungsten; or a transparent electrodesuch as indium tin oxide (ITO).

The first oxide layer 106 a, 106 b of the first and second pixelcircuits 100 a, 100 b is disposed along adjacent sidewalls of theepitaxial substrate layer 116 that face the deep trench isolation region102 to isolate the conductive material 104 a, 104 b from the first andsecond epitaxial substrate layers 116 a, 116 b. The conductive material104 a, 104 b forms a capacitor surface that is thereby used to form acapacitor for an adjacent photodector and is connected to the groundbias potential through a via 114.

The vertical orientation of the capacitor allows for a large chargestorage capacity and high dynamic range without sacrificing the pixelarea exposed to light. This results in smaller sized pixels and higherquantum efficiency providing improved sensor performance and lower cost.

Overall capacitance of a given pixel circuit 100 may be increased byextending the conductive material 104 along an additional sidewall ofthe epitaxial substrate layer 116. For example, with particularreference to FIG. 3, the first conductive material 104 a may form afirst and second capacitor surface that extends continuously along twoadjacent sidewalls of the first epitaxial substrate layer 116 a facingthe deep trench isolation region 102. Using two adjacent sidewallsincreases the overall surface area that can be used to form thecapacitor allowing for a higher capacitance than could be achieved byusing a single sidewall in the deep trench isolation region 102. Morespecifically, the capacitor formed on the first pixel circuit 100 a maycomprise a first capacitor surface extending along the first oxide layer106 a on a first sidewall of the first epitaxial substrate layer 116 aand a second capacitor surface extending along the first oxide layer 106a on a second sidewall of the first epitaxial substrate layer 116 a.Similarly, the capacitor formed on the second pixel circuit 100 b may beformed along a first and second sidewall of the second epitaxialsubstrate layer 116 b substantially identical to the capacitor formed onthe first pixel circuit 100 a.

The first oxide layer 106 may comprise any suitable material forisolating the photodetector from the capacitor such as SiO₂, however,significant improvements to the scene capture range may be achievedthrough the use of more advanced high k dielectric (HiK) materials suchas: halfnium oxide (HfOx); zirconium oxide (ZrOx), zirconium aluminate(ZrAlOx), tantalum pentoxide (Ta₂O₅); and aluminum oxide (Al₂O₃).Similarly, the thickness of the first oxide layer 106 may have an impacton the level of improvement to the dynamic range. The use of HiKmaterials may generally improve the dynamic range but the level ofimprovement may be, at least partially, dependent on the thickness ofthe HiK layer. In the present example, the thickness of the HiK layerand the level of dB improvement to the dynamic range may be inverselyproportional such that as the thickness of the HiK layer increases thedB improvement to the dynamic range may decrease. For example, in oneembodiment, the first oxide layer 106 may extend along two adjacentsidewalls as described above and comprise a thickness of approximately50 nanometers and provide an overall improvement to the dynamic range ofabout 14 dB compared to a similar structure using non-HiK oxidematerials. The dB improvement may increase in a non-linear manner as thethickness of the HiK layer is reduced. Conversely, dB improvement over abaseline figure may decrease as the thickness of the HiK layer isincreased.

In an alternative embodiment, and referring now to FIGS. 4 and 5, theoverall capacitance of a given pixel circuit 100 may be increased evenfurther by extending the conductive material 104 along not only a secondsidewall of the epitaxial substrate layer 116 but also along the uppersurface of the epitaxial substrate layer 116 itself. This createsadditional surface area over which to form the capacitor allowing forincreased capacitance. For example, a first conductive material 104 amay extend along two adjacent sidewalls of the first epitaxial substratelayer 116 a facing the deep trench isolation region 102 and may also beextended along the upper surface of the first epitaxial substrate layer116 a between the second oxide layer 118 a and the silicon layer 112.Similarly, a second conductive material 104 b may extend along twoadjacent sidewalls of the second epitaxial substrate layer 116 b facingthe deep trench isolation region 102 and may also be extended along theupper surface of the second epitaxial substrate layer 116 b between thesecond oxide layer 118 b and the silicon layer 112. This structure maybe repeated for every pixel circuit 100 in the array 300.

Extending the conductive material 104 to the upper surface requires theuse of a transparent electrode to prevent photons passing through themicrolens 110 and color filter 108 from being blocked from entering thephotodetector portion of the pixel circuit 100. For example, in oneembodiment, the first and second conductive materials 104 a, 104 b mayformed from a single layer of ITO that is etched to isolate individualpixel circuits 100 from each other.

As described above, using HiK materials may provide increased advantagesrelating to overall dynamic range improvement. For example, in additionto extending the capacitor to the upper surface of the pixel circuit100, using HiK materials for both the first and second oxide layers 106,118 may provide a dB improvement of between 10 dB and 18 dB as comparedto the same structural formation using other oxide materials.

In an alternative embodiment, and referring now to FIG. 6, capacitanceof a given pixel circuit 100 may also be increased by using twodifferent conductive materials 602, 604 to form the capacitor. Forexample, the capacitor may be formed by filling with deep trenchisolation region 102 with a first conductive material 602 and forming anupper surface from a second conductive material 604. Because the secondconductive material 604 is positioned on the upper surface of theepitaxial substrate layer 116, use of a transparent electrode such asITO is required. Since the first conductive material 602 isn't subjectedto photons in the same manner as the second conductive layer 604, othermaterials such as copper or tungsten may be used. As described above,using HiK materials to form the first and second oxide layers 106, 118may also provide increases to the overall dynamic range capability ofthe pixel circuits 100 a, 100 b.

In yet another alternative embodiment, the conductive material disposedwithin the deep trench isolation region 102 may comprise more than onematerial. For example, and referring now to FIG. 7, the deep trenchisolation region 102 may be formed as a partial deep trench from thefrontside and a subsequent partial deep trench from the backside whichwill connect to the partial deep trench from the frontside. The portionof deep trench isolation region 102 formed from the backside maycomprise a first conductive material 104 a, 104 b extending along afirst portion of the sidewall and the portion of deep trench isolationregion 102 formed from the front side may comprise a second conductivematerial 704 a, 704 b extending along a frontside oxide layer 706 a, 706b that is disposed along a second portion of the sidewall below thefirst oxide layer 106 a, 106 b. The first conductive material 104 a, 104b may comprise metal suck as ITO or tungsten and the second conductivematerial 704 a, 704 b may comprise a different metal based conductor asthat of the first conductive material 104 a, 104 b such as polysilicon,tungsten, or copper. As described above for the first conductivematerial 104, the second conductive material 704 may form a thirdcapacitor surface for the capacitor. The frontside oxide layer 706 a,706 b may comprise any suitable material such as SiO₂.

Another alternative, and referring now to FIG. 8 is that a partial frontside portion of the deep trench isolation region 102 acts only as atrench isolation for holes and is filled with SiO₂. The SiO₂ may alsoextend completely over the second oxide layer 118 a, 118 b and act asthe silicon layer 112.

According to various embodiments of the technology and referring to nowFIGS. 9A-9G, a pixel circuit 100 may be formed by initially etching adeep trench isolation region 102 to separate the epitaxial substratelayer 116 of individual pixel circuits 100 from each other (FIG. 9A).For a BSI wafer the deep trench etch will stop on the shallow trenchisolation oxide (STI) or any insulating layer (e.g. inter leveldielectric like SiO2) that make up the front side circuit. The sidewallsof the epitaxial substrate layers 116 a, 116 b, 116 c, 116 d facing thedeep trench isolation region 102 and the upper surface of each epitaxialsubstrate layer 116 may then be lined with an oxide layer 106, 118 toisolate the individual epitaxial substrate layers 116 (FIG. 9B). Forexample, the oxide layer lining the sidewalls and the upper surface ofthe epitaxial substrate layer 116 may comprise a HiK material such ashafnium oxide having a thickness of between about 15 and 70 nanometers.

The deep trench isolation region 102 may then be filled with aconductive material 104, such as a transparent electrode so that the topsurface of the oxide layer 118 is also covered by the conductivematerial 104 (FIG. 9C). A top surface of the conductive material 104 maythen be etched in a desired pattern 702 to isolate portions of theconductive material 104 to a particular epitaxial substrate layer 116(FIG. 9D). For example, the conductive material may be etched in mannersuch that a first epitaxial substrate layer 116 a is essentiallysurrounded by the conductive material 104. Similarly, a second and thirdepitaxial substrate layer 116 b, 116 c may be isolated by other portionsof the conductive material 104. Each individual portion/section of theconductive material 104 may be coupled to a via 114 a, 114 b, 114 c tocreate a capacitor on each individual pixel circuit 100 a, 100 b, 100 c.

A passivation layer may then be formed over the top of the conductivematerial 104. More specifically, an silicon layer 112 comprising amaterial such as SiO₂ may be formed over the conductive material tocomplete a backside illumination structure on the wafer (FIG. 9E). Acolor filter 108 may then be formed over the silicon layer 112 (FIG. 9F)before a micro lens 110 a, 110 b, 110 c is positioned above eachepitaxial substrate layer 116 a, 116 b, 116 c (FIG. 9G) completingformation of the image sensor.

In the foregoing, the deep trench etch is from the BSI side of thewafer. However, the deep trench isolation region 102 may be formed fromthe FSI side of the wafer. In this case Polysilicon can be used as theconductive material 104 inside the deep trench isolation region 102. Thetrench etch may be timed and stop at a certain depth. Alternativelysilicon on insulator (SOI) wafer may be used which would provide a SiO₂etch stop for the deep trench etch.

In the foregoing description, the technology has been described withreference to specific exemplary embodiments. The particularimplementations shown and described are illustrative of the technologyand its best mode and are not intended to otherwise limit the scope ofthe present technology in any way. Indeed, for the sake of brevity,conventional manufacturing, connection, preparation, and otherfunctional aspects of the method and system may not be described indetail. Furthermore, the connecting lines shown in the various figuresare intended to represent exemplary functional relationships and/orsteps between the various elements. Many alternative or additionalfunctional relationships or physical connections may be present in apractical system.

The technology has been described with reference to specific exemplaryembodiments. Various modifications and changes, however, may be madewithout departing from the scope of the present technology. Thedescription and figures are to be regarded in an illustrative manner,rather than a restrictive one and all such modifications are intended tobe included within the scope of the present technology. Accordingly, thescope of the technology should be determined by the generic embodimentsdescribed and their legal equivalents rather than by merely the specificexamples described above. For example, the steps recited in any methodor process embodiment may be executed in any order, unless otherwiseexpressly specified, and are not limited to the explicit order presentedin the specific examples. Additionally, the components and/or elementsrecited in any apparatus embodiment may be assembled or otherwiseoperationally configured in a variety of permutations to producesubstantially the same result as the present technology and areaccordingly not limited to the specific configuration recited in thespecific examples.

Benefits, other advantages and solutions to problems have been describedabove with regard to particular embodiments. Any benefit, advantage,solution to problems or any element that may cause any particularbenefit, advantage or solution to occur or to become more pronounced,however, is not to be construed as a critical, required or essentialfeature or component.

The terms “comprises”, “comprising”, or any variation thereof, areintended to reference a non-exclusive inclusion, such that a process,method, article, composition or apparatus that comprises a list ofelements does not include only those elements recited, but may alsoinclude other elements not expressly listed or inherent to such process,method, article, composition or apparatus. Other combinations and/ormodifications of the above-described structures, arrangements,applications, proportions, elements, materials or components used in thepractice of the present technology, in addition to those notspecifically recited, may be varied or otherwise particularly adapted tospecific environments, manufacturing specifications, design parametersor other operating requirements without departing from the generalprinciples of the same.

The present technology has been described above with reference to anexemplary embodiment wherein the described pixels are formed in thep-type doped semiconductor epitaxial layer with the n+type doped FDwherein the pinned photodiode collects electrons and the hole signal issensed on a capacitor connected to two sidewalls and/or the uppersurface of the epitaxial substrate layer. The capacitance of the pixelcircuit may be increased or decreased by controlling the amount surfacearea along the sidewalls or upper surface that is used to form thecapacitor.

However, changes and modifications may be made to the exemplaryembodiment without departing from the scope of the present technology.These and other changes or modifications are intended to be includedwithin the scope of the present technology, as expressed in thefollowing claims.

The invention claimed is:
 1. A pixel circuit for a solid-state highdynamic range image sensor, comprising: a micro lens; a color filterdisposed immediately adjacent the micro lens; an epitaxial substratelayer disposed adjacent the color filter opposite the micro lens; afirst oxide layer disposed along a first sidewall of the epitaxialsubstrate layer facing a deep trench isolation region; a second oxidelayer disposed along an upper surface of the epitaxial substrate layer;a silicon layer disposed between the second oxide layer and the colorfilter; and a capacitor, comprising: a first capacitor surface extendingalong the first oxide layer of the epitaxial substrate layer facing thedeep trench isolation region; and a second capacitor surface extendingalong at least one of: the first oxide layer disposed along a secondsidewall of the epitaxial substrate layer facing the deep trenchisolation region; and an upper surface of the second oxide layer belowthe silicon layer.
 2. A pixel circuit according to claim 1, wherein thesecond capacitor surface extends along both of the second sidewall ofthe epitaxial substrate layer and the upper surface of the second oxidelayer.
 3. A pixel circuit according to claim 1, further comprising: afrontside oxide layer disposed along a lower portion of the firstsidewall below the first oxide layer; and a third capacitor surfacedisposed along the frontside oxide layer.
 4. A pixel circuit accordingto claim 1, wherein: the first and second oxide layers each comprise ahigh k material; and the capacitor comprises a transparent electrode. 5.A pixel circuit according to claim 4, wherein the high k materialcomprises hafnium oxide.
 6. A pixel circuit according to claim 4,wherein the transparent electrode comprises indium tin oxide.
 7. A pixelcircuit according to claim 1, wherein the second capacitor surfaceextending along the second oxide layer comprises a transparentelectrode.
 8. A pixel circuit according to claim 7, wherein thetransparent electrode comprises indium tin oxide.
 9. A solid-state highdynamic range image sensor, comprising: an array of pixel circuitsisolated from each other by a deep trench isolation region, wherein eachpixel circuit in the array comprises: a micro lens; a color filterdisposed immediately adjacent the micro lens; an epitaxial substratelayer disposed adjacent the color filter opposite the micro lens; afirst oxide layer disposed along a first sidewall and a second sidewallof the epitaxial substrate layer facing a deep trench isolation region;a high k dielectric layer disposed along the upper surface of theepitaxial substrate layer; a silicon layer disposed between the high kdielectric layer and the color filter; and a capacitor, comprising: afirst capacitor surface extending along the first oxide layer on thefirst sidewall; and a second capacitor surface extending along the firstoxide layer on the second sidewall.
 10. A solid-state high dynamic rangeimage sensor according to claim 9, wherein the capacitor furthercomprises a third capacitor surface extending along the upper surface ofthe epitaxial substrate layer between the high k dielectric layer andthe silicon layer.
 11. A solid-state high dynamic range image sensoraccording to claim 10, wherein the third capacitor surface comprises atransparent electrode.
 12. A solid-state high dynamic range image sensoraccording to claim 11, wherein the transparent electrode comprisesindium tin oxide.
 13. A solid-state high dynamic range image sensoraccording to claim 9, wherein: the first oxide layer comprises a high kmaterial; and the capacitor comprises a transparent electrode.
 14. Asolid-state high dynamic range image sensor according to claim 13,wherein the high k material and the high k dielectric layer eachcomprise hafnium oxide.
 15. A solid-state high dynamic range imagesensor according to claim 13, wherein the transparent electrodecomprises indium tin oxide.
 16. A method of forming a capacitor for asolid-state high dynamic range image sensor, comprising: forming a firstcapacitor surface extending along a first oxide layer disposed along afirst sidewall of an epitaxial substrate layer facing a deep trenchisolation region separating a first pixel circuit from a second pixelcircuit; and forming second capacitor surface extending along at leastone of: the first oxide layer disposed along a second sidewall of theepitaxial substrate layer facing the deep trench isolation regionseparating the first pixel circuit from a third pixel circuit; and anupper surface of a high k dielectric layer disposed along an uppersurface of the epitaxial substrate layer.
 17. A method of forming acapacitor according to claim 16, wherein the second capacitor surfaceextends along both of the second sidewall of the epitaxial substratelayer and the upper surface of the high k dielectric layer.
 18. A methodof forming a capacitor according to claim 16, the first and second oxidelayers each comprise a high k material; and the capacitor comprises atransparent electrode.
 19. A method of forming a capacitor according toclaim 18, wherein the transparent electrode comprises indium tin oxide.20. A method of forming a capacitor according to claim 16, furthercomprising: forming a silicon layer disposed between the high kdielectric layer and a color filter; and forming a micro lens along anupper surface of the color filter.